Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COMPOSITE PIPES AND PROCESS FOR MANUFACTURING THE SAME
The present invention relates to composite pipes used
for supplying or transporting water, gas, chemicals, etc.
and to a process for manufacturing the same.
Pipes of plastic such as polyethylene pipes and lead
pipes have so far been used for the purpose of supplying
water or gas or transporting chemicals, etc. However,
these pipes have the following shortcomings. Lead pipes
have an insufficient strength. Plastic pipes are inferior
in resistance to chemical and resistance to radioactivity.
Also, plastic pipes are slightly permeable to water and
gas, and thus inferior in air tightness. Further, because
of decreased in strength at high temperature, it is diffi-
cult to use a plastic pipe to feed hot water or steam at 80
- 100C or higher.
In order to solve this problem, composite pipes have
been conceived which are formed by combining a lead pipe,
which is excellent in resistance to chemicals, resistance
to radioactivity, non-permeability to water, and mechanical
strength at high temperature, with a plastic pipe, which is
excellent in resistance to pressure and corrosion resis-
tance. However, it is difficult to coat the inner wall of
a plastic pipe with lead. Therefore, such composite pipes
have been produced by coating with plastic the outer
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surface of a lead pipe which has a relatively small inner
diameter (e.g. 25 mm) and is thick (e.g. 2 - 5 mm for a
pipe with 25 mm inner diameter).
With this prior art process, the lead pipe to be
coated with plastic has to have a thickness of 2 mm or more
for form retention. (Even if the thickness of the lead
pipe i5 over 2 mm, it is difficult to obtain a lead pipe
having a high roundness and a large inner diameter of e.g.
50 mm or more without any deformation.) This makes the
final prod~ct heavy and expensive. Further, it was
difficult to produce large-diameter composite pipes having
a high roundness with this prior art process.
The composite pipe which is manufactured in ac-
cordance with the present invention is excellent in resls-
tance to chemicals and resistance to radioactivity and in
air tightness, flexibility and mechanical strength at
high temperature and is light in weight and inexpensive
because it has a thin lead layer inside of the outer
pipe. Even at high temperatures of 100 - 200C, the
mechanical strength is maintained at a sufficiently high
level for actual use. Even though the lead layer is
thin, the composite pipe is also excellent in resistance to
pressure, resistance to marring, form retention and corro-
sion resistance thanks to the outer plastic pipe outside of
the lead layer.
Lead ~and lead alloys) as a metal has a melting point
of over 300C and flexibility, and particularly a high
elongation of ~5 - 50 % against about 33 % for aluminum
which is generally used as flexible pipes (and as a metal
sheath for power cables). Thanks to the high elongation,
aluminum can be uniformly extended easily by applying a
tensile force at relatively low temperature from normal
temperature to e.g. 150C. Lead is an extremely peculiar
metal in this point. But, lead is much poorer than alumi-
num in mechanical strength (lead has a tensile strength of
1.8 - 2.5 kg/mm2 against 8.5 kg/mm2 for aluminum). The
present invention aims to provide extremely useful compo-
site pipes by ulilizing the advantageous properties of
aluminum to expand a lead pipe and by using a plastic pipe
as an outer pipe to compensate for poor mechanical strength
of lead.
Plastic pipes are apt to deform and excellent in
restorability and resistance to bending. But, for metal
pipes such as lead pipes, there is a limit for the wall
thickness in relation to a given inner (or outer) diameter
in order to ensure that buckling does not occur. The
relationship between them is represented by the following
L~Go Brazor's equation:
_ 3 _
~Çi()37~
- 3r4 K3 _ 2J-t - ~
2t 9r2
wherein t: wall thickness of pipe
r: outer radius of pipe
l/K: minimum theoretical bending radius
The graph of Fig. 10 shows the relationship between the
outer diameter of pipe and its wall thickness determined by
use of the L.G. Brazor's equation. The equation means that
the larger the wall thickness, the larger the minimum
allowable bending radius. Thus, the thinner the lead pipe
is, the better. However, in producing lead pipes, the
thinner and the larger in diameter, the more apt to deform
and the more difficult to maintain the roundness the lead
pipes are.
The inventor of the present invention succeeded in
providing composite pipes having a large diameter of e.g.
50 to 150 mm (so far impossible for the abovesaid reason)
and an inner lead pipe as thin as l - 3 mm by arranging a
lead pipe in an outer plastic pipe having a required wall
- thickness and expanding the lead pipe into close contact
with the inner wall of the outer pipe. In other words, he
succeeded in decreasing the wall thickness of the metal
pipe and thus the composite pipe, thereby decreasing the
allowable bending radius and thus improving the flexi-
bility.
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In view of the fact that a major problem in making
the pipes of this type seamless and very elongate was dif-
ficulty in transporting such pipes, decreasing the bending
radius (for winding a pipe around a drum for transportation)
to about 1/25 to 1/30 (in outer radius) means increasiny the
transportable pipe length about twice or more. This marked
effect results from decrease in wall thickness of pipe, as
will be understood from the graph of Fig. 10.
According to one aspect of the present invention
there is provided a process for producing a composite pipe
comprising steps of arranging a lead pipe in an outer pipe
made of plastic with some gap therebetween, and expanding
the lead pipe when inserted into the outer pipe until said
lead pipe expands into close contact with the inner wall of
said outer pipe to form a lead layer thereupon.
In accordance with a further aspect of the present
invention, there is provided a process for manufacturing a
composite pipe which comprises the steps of arranging a
lead pipe in a pipe made of plastic with some gap there-
between, and expanding the lead pipe by sealing a pressure
medium in the lead pipe until it gets into close contact
with the inner wall of the pipe of plastic to form a lead
layer.
For short pipes, a lead pipe may be inserted into a
- 5 -
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pipe of plastic formed beforehand. For elongated pipes, a
lead pipe may be fed to a continuous extruder where a pipe
of plastic is extruded while enclosing the lead pipe as a
core.
With the process according to the present invention,
composite pipes having a thin lead layer on the inner wall
of a pipe of plastic can be easily produced.
other objects and advantages of the present invention
will become apparent from the following description taken
with reference to the accompanying drawings, in which:
Figs. 1 - 5 are sectional views of composite pipes
embodying the present invention;
Figs. 6 - 9 are sectional views corresponding to the
embodiments of Figs. 1 - 4, respectively, but with the lead
pipe not expanded yet; and
Fig. 10 is a graph showing the relationship between
the outer diameter and the thickness of the pipe.
A composite pipe 1 in accordance with the present
invention comprises an outer pipe 2 made of plastic such as
polyethylene and polyvinyl chloride and a thin lead layer 3
disposed in close contact with the inner wall of the outer
pipe, the lead layer 3 being formed by expanding a lead
pipe 4.
In the embodiment of Fig. 1, the pipe 1 is of a circu-
lar section whereas in the embodiment of Fig. 2 it is of a
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square section. Square pipes have advantages that they can
be laid in buildings with a minimum waste of space in
relation to the other structure and that they can be stably
supported on a flat wall surface.
In the embodiment of Fig. 3, the pipe 2 is round, and
is divided by an axially extending partition 8 into two
paths or chambers of a semi-circular section. In each of
these two chambers, a lead layer 3 is formed in close
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contact with the inner wall of the chamber.
In the embodiment of Fig. 4, the pipe 2 is square and
is divided by an axially extending partition 8 into two
chambers or paths of a square section. In each of these
two chambers, a lead layer 3 is formed in close contact
with the inner wall of the chamber.
The embodiment of Fig. 5 is similar to the embodiment
of Fig. 3, but further comprises a heat insulation layer 5
made of a heat insulation material such as glass wool,
asbestos and foamed plastic and arranged outside of the
pipe 2, a protective layer 6 made of aluminum or stainless
steel and arranged outside of the layer ~, and a corrosion
protecting layer 7 made of polyethylene or polyvinyl
chloride and arranged outside of the layer 6. The protec-
tive layer 6 may be corrugated.
The composite pipes 1 according to the present inven-
tion may be produced by the following process.
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Firstly, a lead pipe 4 is extruded by use of a con-
tinuous lead extruder. The lead pipe 4 is then fed to a
continuous synthetic resin extruder by which a pipe 2 made
of plastic such as polyethylene and polyvinyl chloride is
extruded continuously while enclosing the lead pipe 4 as a
core with some space S between the pipe 2 and the lead pipe
4, as shown in Figs. 6, 7, 8 and 9. Both ends of the lead
pipe 4 are then sealed and pressure medium such as gas or
liquid is forced into the lead pipe 4 to expand it into
close contact with the inner wall of the pipe 2. Simul-
taneously air in the space between the pipe 2 and the lead
pipe 4 is extracted by vacuum.
In expanding the lead pipe 4, it should be heated
beforehand to a temperature of 150C or less. If it was
heated to a higher temperature, the strength of the lead
pipe could deteriorate and its locally weak points could
get damaged by the pressure applied from inside. Also,
such a high temperature could deteriorate the outer pipe 2
of plastic.
The internal pressure applied for expanding the lead
pipe 4 should be not higher than 50 kg/cm2, so that the
outer pipe 2 will not be deformed even if it has a large
diameter. The highest allowable pressure should be
selected by use of the known equation (P = ~,t wherein
P: internal pressure in kg/cm2, t: wall thickness in cm,
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r': inner radius in cm, ~: allowable stress in kg/cm2) so
that it will be below the breaking tensile strength.
As for the manner of applying the internal pressure,
because applying it to a final pressure at a time could
damage the locally weak points of the lead pipe, it is
preferable to increase the internal pressure step by step
up to the allowable highest pressure, e.g. 5 kg/cm2 for one
hour, then 10 kg/cm2 for one hour, and then 15 kg/cm2 for
one hour.
If the composite pipe 1 is relatively short, it may be
produced by forming the outer pipe 2 and the lead pipe 4
separately beforehand, inserting the lead pipe into the
outer pipe, and expanding the lead pipe.
Composite pipes of a square or two-path cross-section
as shown in Figs. 2, 3 and 4 have been very difficult to
manufacture. In accordance with the present invention,
such composite pipes can be easily manufactured to any
desired length by firstly forming a lead pipe or pipes 4 of
a circular section, extruding a pipe 2 of plastic by means
of an extruder while enclosing the lead pipe 4 inserted as
a core, as shown in Figs. 7, 8 and 9, and filling the lead
pipe 4 with pressure medium to expand it into close contact
with the inner wall of the outer pipe 2.
By the abovesaid process, for example, a lead pipe 4
having a thickness of 2 mm can be expanded to form a thin
~2~)3~5
lead layer having a thickness of 1 mm or less in contact
with the inner wall of the outer pipe 2.
The material of the outer pipe is not intended to
limit to polyethylene and polyvinyl chloride. It may be
polybutene, crosslinked polyethylene, high-density poly-
ethylene, polypropyrene, heat-resistant polyvinyl chloride,
polymethylpentene, etc. These plastics may be used
according to the use. The material of the outer plastic
pipe should be selected in view of the cost, working tempe-
rature, resistance to external pressure such as soil
pressure, and heat insulation. As for the working tempe-
rature, polyethylene, polyvinyl chloride and high-density
polyethylene are preferable for low and medium temperature,
e.g. -50~C to 80C, and polybutene, cross-linked poly-
ethylene, polypropyrene, heat-resistant polyvinyl chloride
and polymethylpentene are preferable for use at high tempe-
rature such as 80 - 120C. As for heat insulation, poly-
butene is superior to polyethylenes about twice. Poly-
ethylene and polyvinyl chloride are almost the same, but
the latter is slightly better.
Since the outer pipe is made of plastic, the
composite pipe of the present invention can be manufactured
by extrusion in the form of a seamless elongated pipe. The
composite pipe as a whole has an excellent i-lexibility
because it is seamless and thin. Thus, it can be trans-
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ported wound around a drum for a larger length and can belaid bent with a smaller radius of curvature.
Thus, the composite pipe according to the present
invention can be used for a variety of applications such as
at oil producing plants,-as water pipes or steam pipes, as
containers of elongated heat pipes, as freezing preventive
pipes, in ships, at atomic power plants, in factories, etc.
With the process according to the present invention,
since the lead layer is formed by expanding a lead pipe, a
thin lead layer can be formed stably and seamlessly to the
shape of the outer pipe. Since the outer pipe and the lead
pipe can be formed by extrusion, composite pipes having
a large diameter and a high roundness can be produced.
Also, composite pipes having a square cross-section or
having two chambers extending axially can be produced.
The process according to the present invention makes a
good use of the advantageous properties of lead: high
elongation, uniformly expansible at relatively low tempe-
ratures under a relatively small internal pressure, and
intrisically flexibleD
In the process o~ the present invention, the wall
thickness of the composite pipe can be decreased by expand-
ing a lead pipe arranged in an outer pipe. This makes it
possible to decrease the bending radius, thereby decrease
the diameter of the drum on which the pipe is wound,
~2~i~375
increase the length of pipe which can be transported, and
transport a very long pipe. Also, the decrease in bending
radius has a great effect in laying the pipe.
